1. Field of the Invention
The present invention relates to apparatus for forming an image (on a recording medium or for viewing) from an electrical signal and more particularly to improvements in electronic imaging apparatus of the kind using an array of individually addressable light valves.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,229,095 discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. Thus an array of such light valves comprises a panel of PLZT material with a plurality of interleaved electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing of such light valves in synchronization with the energization of red, green and blue exposing sources and according to the red, green and blue color information for the pixels of that image.
It is highly desirable for such light valve imaging apparatus to have a contrast ratio (i.e. the magnitude of light transmitted when the light valve is "on" to the magnitude of light transmitted when the light valve is "off") which is high. This holds true even for continuous tone imaging, for one preferred technique for achieving a grey scale (i.e. a variation in densities recorded on the photosensitive recording medium) utilizes fixed intensity and variable time to achieve varying exposure levels. I have found that the contrast ratio of such light valve imaging apparatus is significantly affected by the nature of illumination applied to the light valve array, and it is a significant purpose of the present invention to provide improved structure and technique for controlling the light valve illumination to increase the light valve's contrast ratio.